This invention relates to the field of communications, and more particularly to systems in which optical signals are switched among multiple network nodes communicating via fiber optic cables.
Optical switches for switching of optical communications signals among communicating nodes are generally known in the art. For example, U.S. Pat. No. 4,365,863 to Broussard (1982) discloses a communications switching system comprising two optical fiber arrays facing one another, an input array and an output array. Optical beams are formed such that each beam contains the signals from a single input fiber. Each beam is deflected toward a signal output fiber and concentrated onto that fiber. For each pair of input/output fibers a separate mechanism for beam forming, conversion, deflecting, and control is used.
Two papers have been authored by the inventor Kevin DeMartino that describe techniques for optical switching in which each node is connected to the switch by a cable containing multiple optical fibers. One paper is entitled "Processing of Synchronous Communications Signals", and the other is entitled "STM Versus ATM Signal Processing". These papers were submitted in the Proceedings of the 6.sup.th and 7.sup.th International Conferences on Signal Processing Applications and Technology (ICSPAT) respectively.
In switches described in those papers, optical beams are formed such that within each beam the input signals from a particular cable are spatially adjacent to each other. The signals within each beam are gated and shifted to the appropriate output. With this approach, signals from multiple adjacent input fibers can be switched as a group. The fibers included in each group can be dynamically assigned. The characteristics of the optical signals are unaltered as the signals flow through the switch and the associated data rates are not limited by the switch. This technique enables switching as a group those signals contained in multiple adjacent fibers, rather than individually switching the signals associated with each fiber. The number of groups of fibers, which is related to the number of nodes that must be connected together, can be significantly less than the number of individual fibers. Consequently, with the described technique, the number of switching elements and the overall complexity of the switch can be significantly reduced.
With the techniques described above all the signals within an optical fiber are switched intact; the signals within a particular fiber are all routed to the same destination. The input fibers within a cable are spatially separate and can be viewed as space channels. Thus in the above-described techniques an integral number of space channels are assigned to each connection through the switch. This results in coarse quantization of assigned channels. The channel utilization is low unless the data rates are very high and there are several space channels per connection. Also, the number of fibers within a cable and the number of space channels is limited. Consequently, the number of nodes that can be connected to the switch is limited by the number of fibers per cable and the number of fibers used by each node to communicate with the other nodes.
An additional drawback of the above-described techniques appears in larger networks, in which switches are used in combination to form a network or segment of a network. A network segment composed of switches similar to the switches described above typically requires many more space channels than an individual switch. Multiple space channels must be provided for each connection through the network segment. This restriction limits the size of the network segment and the number of nodes that can be connected to it.